Trace-gas sensing is a rapidly growing field of research and has received considerable attention, especially in the detection and quantification of greenhouse gases (e.g., N2O and CO2). It has also applications in non-invasive medical diagnostics, environmental monitoring, and homeland security.
Fertilizers used in agricultural fields are major sources of N2O. The use of fertilizer will increase in the next few decades to meet the demand of food production as the global population increases. However, optimizing the efficiency with which fertilizers produce nutrients, combined with the design of new forms of fertilizers, can reduce their emission of N2O. It is also important to note that the excess fertilizer drains into rivers and lakes due to rain or irrigation and polluting water bodies [1]. Further, the emission of N2O is spatially variable in soil because of soil factors that lead to the production, consumption, and mobility of the gas. Thus the ability to pin-point “hot spots” of N2O emissions will allow one to mitigate soil factors. A farmer can regulate the use of fertilizer by measuring the concentration of N2O emitted from agricultural fields due to the application of fertilizer at different climate and soil conditions. This ability will facilitate the fertilizer industry's worldwide program of 4R (right fertilizer source, right rate, right time and right replacement) Nutrient Stewardship management, which will in turn improve farm management and finally reduce greenhouse gas emissions.
The current widely used technologies (e.g. GC: Gas Chromatograph, FTIR: Fourier Transform Infrared spectroscopy, laser spectroscopy using a lead-salt detector cooled by liquid nitrogen or thermoelectric cooler, and cavity ring down spectroscopy using a quantum cascade laser) to detect trace gases are complex and expensive [2]. Thus, a compact and cost effective system that can operate at room temperature is in demand. A number of important gases (e.g. CH4, NH3, C2H2, H2S, N2O and CO2) have overtones of the characteristic absorption (fundamental) and the combinations of the overtones bands in the near-infrared (NIR) region (1-2 μm) of the electromagnetic spectrum, which matches the emission spectrum of rare-earth (e.g. Erbium) doped fiber [3]. This makes it possible to use passive and active optical components available from telecom industries to develop a compact and cost-effective device for the detection of trace gases.
The greenhouse effect is caused by the absorption of infrared radiation (IR) from sunlight by gases such as nitrous oxide (N2O). Qualitatively, gases can be differentiated by their absorption lines, and quantitatively, their concentrations can be determined by measuring the degree of absorption of light directed through a gas sample. The absorption of electromagnetic radiation (e.g. IR or NIR) by a gas is governed by the Beer-Lambert Law [4]:
                              I                      I            0                          =                  exp          ⁡                      (                                          -                α                            ⁢                                                          ⁢              CL                        )                                              (        1        )            
where I0 is the intensity of the incident optical radiation, I is the transmitted optical intensity, a is the absorption coefficient of the gas molecules (an important parameter dependent on both the gas species and the wavelength of incident optical radiation), C is the concentration of the absorbing molecules and L is the optical path length of the gas cell or absorption path length. In general, absorption spectroscopy (e.g. FTIR) makes use of incoherent light sources such as incandescent bulbs to generate IR radiation. These sources are essentially blackbody radiators, and complex optical components are required to collimate and direct the beam through the sample with narrow bandwidth. The sensitivity of the above devices is limited by the physical length of the gas cell. Highly sensitive spectroscopic techniques to enhance the absorption path length have been developed based on the laser, such as continuous-wave cavity ring-down spectroscopy (CW-CRDS) and intracavity laser absorption spectroscopy (ICLAS) [5]. The conventional CRDS technique involves measuring the decay time of the laser pulse injected into a high finesse cavity (Fabry-Perot or Ring configuration) that contains the gas sample, where the rate of decay of the pulse indicates the absorption by the gas sample. One can calculate the concentration of the gas sample from the decay time or the ring-down time. On the other hand, in ICLAS, the gas cell is used inside the laser cavity and no external laser is required. Both CRDS and ICLAS increase the effective absorption length by several times, compared to conventional FTIR systems [5]. As the path length is enhanced, the sensitivity of the device increases. Thus, combining advanced detection techniques with a gas cell with longer optical path length makes it possible to develop a very highly sensitive gas detection system.
In the present Application, details are provided concerning the design of novel gas cells, and their application for the detection of greenhouse gas; more specifically, nitrous oxide (N2O).